586 PHYSIOLOGY OF THE NERVE CELL AND THE SPINAL CORD 



A. CONTROL OF SKELETAL MUSCLES 



The independent influence of the spinal cord upon movements of the 

 skeletal muscles is primarily of a reflex nature. Under suitable conditions 

 all the muscles innervated from the cord can be thrown into action by stimu- 

 lation of a single afferent nerve, even when the cord is isolated from the 

 brain. These reflexes are in general well coordinated, and, as the examples 

 given below will prove, are unquestionably purposive in character. Be it 

 observed, however, that these phenomena are not to be regarded as the ex- 

 pression of a "conscious" activity on the part of the cord. We ourselves 

 often perform much more complicated movements without being in any wise 

 conscious of them. 



When a drop of sulphuric acid is placed on the leg of a decapitated frog-, the 

 animal tries to remove the irritant with the same leg. But if this leg be held 

 fast, the other hind leg comes to the help of the first. When a toe is pinched 

 lightly the leg is drawn up against the abdomen. But when the drop of acid 

 is placed near the anus, both legs are drawn up and are then powerfully extended. 



When the foot sole of a dog, whose spinal cord has been severed, is gently 

 pressed against with a broad surface the leg makes a strong extensor movement. 

 But if the same foot sole be touched with a sharp point, a flexor movement 

 is made, as if the animal wished to withdraw the foot from a painful impulse 

 (Sherrington). 



Observations on the ability of the spinal cord, isolated from the brain and 

 medulla, to regulate the muscular movements necessary for locomotion are of 

 particular interest. An eel deprived of its head immediately after the operation 

 swims about in a basin, behaving' just like a normal fish. It does not merely 

 writhe about on the bottom, but swims up and down and about through the 

 water in all directions. But the beheaded eel is not able to maintain its normal 

 position in the water and can no longer swim backward. 



Schrader found that the entire medulla of the frog as far up as the tip of 

 the calamus scriptorius can be removed without destroying the locomotor reac- 

 tion to reflex stimuli. The movements were rather awkward but were nevertheless 

 perfectly coordinated. Frog tadpoles and very young frogs exhibit movements 

 of the hinder parts without any external stimulus (Babak). 



It is a very old observation that decapitated chickens can still fly. And 

 ducks with the spinal cord severed between the fourth and fifth cervical verte- 

 brae make perfectly regular and very energetic swimming movements with their 

 feet, even when not stimulated externally; they make steering movements with 

 the tail and flying movements with the wings, etc. But when set down on a 

 table they can neither maintain their equilibrium nor walk (Tarchanoff). 



So far as is known to the author nobody has ever yet observed movements of 

 locomotion, either spontaneously or reflexly produced, in decapitated mammals. 

 We conclude that in the lower vertebrates at least the spinal cord is of itself to 

 a greater or less extent able to regulate the muscular contractions of locomotor 

 movements. 



The investigations of Sherrington, Hering, Jr., and Biedermann have given 

 us some very important information as to the mechanism concerned in these 

 reflexes. . . 



The first effect of a brief, weak stimulation of the web of a frog's foot after 

 the spinal cord has been isolated, is always a flexor movement on the same side 

 or an extensor movement on the other side, in case the flexor movement is pre- 



